DISCUSSTHECONSTRUCTIONTEMPERATUREANDCRACKOFTHECONCRETELIGHTLY淺談混凝土的施工過程中溫度與裂縫控制資料來源THEAMERICANSOCIETYOFCIVILENGINEERS設計題目長春市外貿局辦公樓結構、施工設計學院名稱土木工程學院專業名稱土木工程班級名稱土木1043完成時間2014年3月25日DISCUSSTHECONSTRUCTIONTEMPERATUREANDCRACKOFTHECONCRETELIGHTLYTHECONCRETEOCCUPIESTHEIMPORTANTPOSITIONINMODERNENGINEERINGCONSTRUCTIONBUTTODAY,THECRACKOFTHECONCRETEISCOMPARATIVELYGENERAL,THECRACKSARENEARLYOMNIPRESENTINTHESCIENCEOFBRIDGEBUILDINGTHOUGHWETAKEVARIOUSKINDSOFMEASURESINCONSTRUCTING,CAREFUL,BUTTHECRACKSTILLOCCURSNOWANDTHENTRACINGITTOITSCAUSE,ITISONEOFTHEMINCOMPLETELYTHATOURCHANGETOCONCRETETEMPERATURESTRESSPAYSATTENTIONTOINTHELARGEVOLUMECONCRETE,TEMPERATURESTRESSANDTEMPERATURECONTROLARESIGNIFICANTTHISISMAINLYBECAUSEOFTHEREASONOFTWORESPECTSFIRSTOFALL,CONCRETEOFTENAPPEARTHETEMPERATURECRACKINNOTCONSTRUCTING,INFLUENCETHEGLOBALITYANDDURABILITYOFTHESTRUCTURESECONDLY,INTHECOURSEOFOPERATING,THETEMPERATURECHANGEHASREMARKABLEINFLUENCETHATCANTBEIGNOREDONTHESTRESSSTATEOFTHESTRUCTUREWEMEETTOCONSTRUCTTEMPERATURECRACKINMAINLY,SOONLYTOORIGINCAUSEOFFORMATIONANDTREATMENTMEASURE,CONCRETEOFCRACKMAKEADISCUSSIONINCONSTRUCTINGTHISTEXT1REASONOFACRACKHAVEMANYKINDSOFREASONSTOPRODUCETHECRACKINTHECONCRETE,ITISMAINLYTHECHANGESOFTEMPERATUREANDHUMIDITY,FRAGILITYANDDISPARITYOFTHECONCRETE,ANDTHESTRUCTUREISUNREASONABLE,THERAWMATERIALSISNOTUPTOSTANDARDIFTHEALKALIAGGREGATEREACT,THETEMPLATEISOUTOFSHAPE,THEFOUNDATIONDOESNOTSUBSIDEETCEVENLYTHECEMENTEMITSALARGEAMOUNTOFHEATOFHYDRATIONWHENTHECONCRETEISHARDENNED,INSIDETEMPERATUREISRISINGCONSTANTLY,CAUSETHESTRESSOFDRAWINGONTHESURFACEINTHECOURSEOFLOWERINGTHETEMPERATURE,ISITCONGEALFOUNDATIONPAYRESTRAINTOMIXALWAYSLATERSTAGE,WILLPRESENTTHESTRESSOFDRAWINGWITHINTHECONCRETEREDUCINGOFTEMPERATURECANSURFACECAUSEHEAVYSTRESSOFDRAWINGVERYINCONCRETETOOWHENTHESEDRAWTHESTRESSANDGOBEYONDRESISTINGTHEABILITYOFSPLITTINGOFCONCRETE,NAMELYWILLPRESENTTHECRACKALOTOFINSIDEHUMIDITYOFCONCRETECHANGEVERYLIGHTORCHANGERELATIVELYSLOW,SURFACEHUMIDITYMIGHTCHANGEHEAVYTHEVIOLENTCHANGETAKESPLACINGSUCHASMAINTAININGTHOROUGHLY,WHENGETTINGWETTERWHENNOTDOING,CONTRACTSURFACETHEREARENTDEFORMATIONDOING,OFTENCAUSETHECRACKTOOTHECONCRETEISAKINDOFFRAGILITYMATERIAL,TENSILESTRENGTHISABOUT1/10OFTHECOMPRESSIONSTRENGTH,ISITCARRYONONESSHOULDERORBACKLIMITWHENDRAWOUTOFSHAPETOHAVE104ONLY,ISITCARRYONONESSHOULDERORBACKLIMITLOCATIONWHENSTRETCHOUTOFSHAPETOTHEREIS104TOADDFORALONGTIMETOADDASHORTTIMEBECAUSERAWMATERIALSEVEN,WATERDUSTTHANUNSTABLE,TRANSPORTANDBUILDPHENOMENONOFEMANATINGOFCOURSE,ITSTENSILESTRENGTHISNOTEVENINTHESAMECONCRETE,ALOTOFRESISTTHEABILITYOFDRAWINGVERYLOW,ITISAPTTOPRESENTTHEWEAKPOSITIONOFTHECRACKAMONGARMOREDCONCRETE,DRAWSTRESSTOUNDERTAKEBYREINFORCINGBARMAINLY,CONCRETEBEARSTRESSOFKEEPINGJUSTORREINFORCINGBARMIXIFEDGEPOSITIONGONETOTOCONGEALPRESENTTHESTRESSOFDRAWINGINTHESTRUCTUREINPLAINCONCRETE,MUSTRELYONTHECONCRETEONESELFTOBEARREQUIRETOAVOIDTHESTRESSOFDRAWINGORONLYVERYSMALLSTRESSOFDRAWINGAPPEARSOFTHETHEGENERALDESIGNBUTTHECONCRETEISCOOLEDFROMMAXIMUMTEMPERATURETOTHESTEADYTEMPERATUREOFOPERATINGPERIODINCONSTRUCTING,OFTENCAUSESIZABLETODRAWTHESTRESSWITHINTHECONCRETETHETEMPERATURESTRESSCANEXCEEDOTHEROUTSIDESANDLOADTHESTRESSESCAUSEDSOMETIMES,KNOWCHANGELAW,TEMPERATUREOFSTRESSFORCARRYONREASONABLESTRUCTURALDESIGNANDCONSTRUCTEXTREMELYIMPORTANT2ANALYSISOFTEMPERATURESTRESSESCANBEDIVIDEDINTOFOLLOWINGTHREESTAGESACCORDINGTOTHEFORMINGPROCESSOFTHETEMPERATURESTRESS（1）ITISEARLYBUILDCONCRETEISITISITOVERBASICALLYTOSENDOUTHEATTOCEMENTTOBEGIN,GENERALLYONEONEDAYBYONESELFTWOCHARACTERISTICSATTHISSTAGE,FIRST,THECEMENTEMITSALARGEAMOUNTOFHEATOFHYDRATION,SECOND,MIXANDCONGEALTHECHANGINGSHARPLYOFELASTICMODELQUANTITYBECAUSEOFTHECHANGEOFELASTICMODELQUANTITY,FORMTHEREMAININGSTRESSINTHECONCRETEINTHISPERIOD（2）MIDDLEPERIODUPTILLTHECONCRETEISCOOLEDUNTILSTABILITYTEMPERATUREFROMCEMENTSENDOUTHEATFUNCTIONBASICALLYWHENEXPIRING,INTHISPERIOD,THETEMPERATURESTRESSISMAINLYBECAUSETHECOOLINGOFTHECONCRETEANDEXTERNALTEMPERATURECHANGECAUSE,THESESTRESSESANDREMNANTSSTRESSESTHATISFORMEDINEARLYDAYSARESUPERPOSED,MIXANDCONGEALTHEELASTICMOULDAMOUNTTHATGOESTOANDDOESNOTCHANGEMUCHDURINGTHISPERIOD（3）LATERPERIODOPERATIONPERIODAFTERTHECOMPLETECOOLINGOFCONCRETETEMPERATURESTRESSWHETHEREXTERNALTEMPERATURECHANGECAUSEMAINLY,THESESTRESSESANDFIRSTTWOKINDSOFREMNANTSSTRESSESESARECHANGEDANDADDEDCANBEDIVIDEDINTOTWOKINDSACCORDINGTOTHEREASONWHYTHETEMPERATURESTRESSCAUSES（1）SPONTANEOUSSTRESSTHEREARENOTANYRESTRAINTORTOTALLYSTATICSTRUCTUREATTHEBORDER,IFINSIDETEMPERATUREISNONLINEARDISTRIBUTION,TEMPERATURESTRESSAPPEARINGBECAUSESTRUCTURERESTRAINSFROMEACHOTHERFOREXAMPLE,THEBODYOFMOUNDOFTHEBRIDGE,THEPHYSICALDIMENSIONISRELATIVELYLARGE,SURFACETEMPERATUREISLOWWHENTHECONCRETEISCOOLED,INSIDETEMPERATUREISHIGH,PRESENTTHESTRESSOFDRAWINGONTHESURFACE,PRESENTTHESTRESSOFPRESSINGINTHEMIDDLE（2）RESTRAINTHESTRESSALLOFTHESTRUCTUREONESORITRESTRAINEXTERNALONESOMEBORDER,CANTOUTOFSHAPEANDSTRESSNOTCAUSENOTFREESUCHASCASEROOFBEAMROOFCONCRETEANDGUARDRAILCONCRETETHISTWOKINDSOFTEMPERATURESTRESSESDRAWBACKSTRESSESCAUSEDTOACTONWITHTHEDOINGOFCONCRETETOGETHERFREQUENTLYITISAMORECOMPLICATEDJO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混凝土中產生裂縫有多種原因，主要是溫度和濕度的變化，混凝土的脆性和不均勻性，以及結構不合理，原材料不合格（如堿骨料反應），模板變形，基礎不均勻沉降等。混凝土硬化期間水泥放出大量水化熱，內部溫度不斷上升，在表面引起拉應力。后期在降溫過程中，由于受到基礎或老混凝上的約束，又會在混凝土內部出現拉應力。氣溫的降低也會在混凝土表面引起很大的拉應力。當這些拉應力超出混凝土的抗裂能力時，即會出現裂縫。許多混凝土的內部濕度變化很小或變化較慢，但表面濕度可能變化較大或發生劇烈變化。如養護不周、時干時濕，表面干縮形變受到內部混凝土的約束，也往往導致裂縫。混凝土是一種脆性材料，抗拉強度是抗壓強度的110左右，短期加荷時的極限拉伸變形只有（0610）104，長期加荷時的極限位伸變形也只有（1220）104由于原材料不均勻，水灰比不穩定，及運輸和澆筑過程中的離析現象，在同一塊混凝土中其抗拉強度又是不均勻的，存在著許多抗拉能力很低，易于出現裂縫的薄弱部位。在鋼筋混凝土中，拉應力主要是由鋼筋承擔，混凝土只是承受壓應力。在素混凝土內或鋼筋混凝上的邊緣部位如果結構內出現了拉應力，則須依靠混凝土自身承擔。一般設計中均要求不出現拉應力或者只出現很小的拉應力。但是在施工中混凝土由最高溫度冷卻到運轉時期的穩定溫度，往往在混凝土內部引起相當大的拉應力。有時溫度應力可超過其它外荷載所引起的應力，因此掌握溫度應力的變化規律對于進行合理的結構設計和施工極為重要。2溫度應力的分析根據溫度應力的形成過程可分為以下三個階段（1）早期自澆筑混凝土開始至水泥放熱基本結束，一般約30天。這個階段的兩個特征，一是水泥放出大量的水化熱，二是混凝上彈性模量的急劇變化。由于彈性模量的變化，這一時期在混凝土內形成殘余應力。（2）中期自水泥放熱作用基本結束時起至混凝土冷卻到穩定溫度時止，這個時期中，溫度應力主要是由于混凝土的冷卻及外界氣溫變化所引起，這些應力與早期形成的殘余應力相疊加，在此期間混凝上的彈性模量變化不大。（3）晚期混凝土完全冷卻以后的運轉時期。溫度應力主要是外界氣溫變化所引起，這些應力與前兩種的殘余應力相迭加。根據溫度應力引起的原因可分為兩類（1）自生應力邊界上沒有任何約束或完全靜止的結構，如果內部溫度是非線性分布的，由于結構本身互相約束而出現的溫度應力。例如，橋梁墩身，結構尺寸相對較大，混凝土冷卻時表面溫度低，內部溫度高，在表面出現拉應力，在中間出現壓應力。（2）約束應力結構的全部或部分邊界受到外界的約束，不能自由變形而引起的應力。如箱梁頂板混凝土和護欄混凝土。這兩種溫度應力往往和混凝土的干縮所引起的應力共同作用。要想根據已知的溫度準確分析出溫度應力的分布、大小是一項比較復雜的工作。在大多數情況下，需要依靠模型試驗或數值計算。混凝土的徐變使溫度應力有相當大的松馳，計算溫度應力時，必須考慮徐變的影響，具體計算這里就不再細述。3溫度的控制和防止裂縫的措施為了防止裂縫，減輕溫度應力可以從控制溫度和改善約束條件兩個方面著手。控制溫度的措施如下（1）采用改善骨料級配，用干硬性混凝土，摻混合料，加引氣劑或塑化劑等措施以減少混凝土中的水泥用量；（2）拌合混凝土時加水或用水將碎石冷卻以降低混凝土的澆筑溫度；（3）熱天澆筑混凝土時減少澆筑厚度，利用澆筑層面散熱；（4）在混凝土中埋設水管，通入冷水降溫；（5）規定合理的拆模時間，氣溫驟降時進行表面保溫，以免混凝土表面發生急劇的溫度梯度；（6）施工中長期暴露的混凝土澆筑塊表面或薄壁結構，在寒冷季節采取保溫措施；改善約束條件的措施是（1）合理地分縫分塊；（2）避免基礎過大起伏；（3）合理的安排施工工序，避免過大的高差和側面長期暴露；此外，改善混凝土的性能，提高抗裂能力，加強養護，防止表面干縮，特別是保證混凝土的質量對防止裂縫是十分重要，應特別注意避免產生貫穿裂縫，出現后要恢復其結構的整體性是十分困難的，因此施工中應以預防貫穿性裂縫的發生為主。在混凝土的施工中，為了提高模板的周轉率，往往要求新澆筑的混凝土盡早拆模。當混凝土溫度高于氣溫時應適當考慮拆模時間，以免引起混凝土表面的早期裂縫。新澆筑早期拆模，在表面引起很大的拉應力，出現“溫度沖擊”現象。在混凝土澆筑初期，由于水化熱的散發，表面引起相當大的拉應力，此時表面溫度亦較氣溫為高，此時拆除模板，表面溫度驟降，必然引起溫度梯度，從而在表面附加一拉應力，與水化熱應力迭加，再加上混凝土干縮，表面的拉應力達到很大的數值，就有導致裂縫的危險，但如果在拆除模板